Stations make use of a unique station address, referred to as "my address" to identify the station. (Quote marks are used herein to set off compound nouns when their use helps to clarify the meaning of a sentence, as in "my address". Also quote marks are used to set off other words when their use helps to clarify the meaning of the sentence.) Stations also maintain a list of address which may contain my address for the purpose of comparing an address in the destination address field of a frame detected on the network. In the event that a station finds a match between the destination address of a frame and an address in the list of addresses, the station takes predetermined actions.
Also in most token controlled networks, the station transmitting a frame onto the network is required to remove, or "strip", the frame from the network after the frame circumnavigates the network and returns to the originating station. In some networks, stripping is accomplished by a station detecting a frame on the network, comparing the source address SA of the frame with the my address of the station, and stripping the frame in the event that the SA matches the my address. This form of stripping is called "source address stripping".
As the result of a fault, a station may transmit a frame onto the ring where the source address SA field of the frame is the same as the my address of another station of the network. For example, a software or hardware fault in a station may cause the fault. Or, for a further example, a human error in entering a station address may have resulted in a duplicate address on the network. The fault of having a duplicate station address on a single local area network is referred to as the "duplicate address problem".
In some cases the duplicate address fault may have little or no effect on the network, where in other situations the fault condition may destroy the operability of the network.
A ring network may have two states, a "ring operating state", and a "ring not operating state". When the ring is in the "ring not operating state", the ring may be in recovery and initialization process. A typical token ring protocol may use Beacon, Claim, or other MAC (Medium Access Control), frames for recovery and initialization.
In the event that two stations have the same my addresses, or the event that one station has a list of address and the list contains an address the same as the "my address" of another station on the network, then when the network is in the "ring not operating state" the network may fail to become operational. Alternatively, after the ring has been in the "ring operating" state, the ring may cease to operate after a period of successful operation.
More details of network failure modes resulting from duplicate address situations are discussed in the APPENDIX herebelow. Also details of the Fiber Distributed Data Interface, FDDI, ring network along with duplicate address failure modes are discussed in the APPENDIX.
Also discussed in the Appendix is the ANSI/FDDI Ring Management protocols, referred to as RMT. RMT may detect duplicate "my addresses" on a communications ring. RMT requires executing complex logic in order to accomplish detection of duplicate "my addresses". Accordingly, RMT is too time consuming to execute in the event that a source address in a frame is compared with a list of addresses in the station. The difficulty level, such as the time required to operate RMT for a source address in a frame with a list of addresses, or to implement the logic for RMT for a list into a silicon chip, increases approximately proportionally to the number of addresses in the list. For a list of reasonable length the time to execute the RMT logic may be too long for ordinary operation of the station.
The detection of duplicate addresses during ring recovery and initialization has not been satisfactorily solved, and so rings may enter a series of oscillations and fail to become operational.